Will wearing glasses make my eyesight worse?

Q. I'm starting to have trouble reading. But I've heard that wearing glasses to help me read will make my eyesight worse. Is that true?

A. This is a common question. Many people believe that glasses can make eyesight worse, but that's more myth than reality.

Many of us start to have trouble reading in our mid-40s. The condition is called presbyopia (pronounced prez-bee-OH-pee-ah), and it is the natural loss of the focusing ability of the lens of the eye.

The lens is about the size of a shirt button. Because it can change shape, we are able to see objects that are close or far away. The closer the object, the more the lens has to "flex" to bring the object into focus. With age, though, the lens slowly grows larger and thicker. As it grows, the many tiny ligaments that connect the lens to the ciliary, or "focusing," muscle in the eye become slack. When that happens, those ligaments (called zonules) cannot exert enough force on the lens to bend it into the position necessary to see things clearly up close. The lens also hardens and becomes less flexible, compounding the problem.

Exercising the ciliary muscles so they could pull harder on the lens would seem logical, but these muscles don't get appreciably weaker with age. Even if eye exercises could strengthen the ciliary muscles, they wouldn't have much effect.

There are two reasons people wrongly blame glasses for worsening presbyopic vision. First, the underlying condition worsens during the period when they start wearing reading glasses, so they associate the glasses with declining vision. Second, they get used to seeing near objects well when wearing reading glasses, so when they take them off, their vision seems to have gotten worse. They blame the reading glasses, when they're really just experiencing the contrast between corrected and uncorrected vision.

People do learn to cope with bad eyesight. The brain learns how to interpret blurry images and make educated guesses. If glasses make it easier for you to see well, your brain may get out of practice doing the tricks it learned to do to cope with poor eyesight. But that's not the same thing as glasses making your eyesight worse.

Whether glasses worsen presbyopia is a settled issue. They don't. But it's not as clear-cut when it comes to childhood myopia, or nearsightedness (a term that causes confusion: it means your sight is good for near things and bad for things in the distance). The National Eye Institute has reported that the prevalence of myopia has increased by 66 percent since 1980, and this increase has prompted many studies aimed at understanding the cause of myopia.

Myopia causes distant objects to be blurry because the eye grows too long, so the focal point of the lens ends up in front of the retina instead of directly on it. The condition usually develops in childhood and gradually worsens until eye growth slows down in early adulthood.

Myopia is largely genetic, but the progression of the condition may be influenced by environmental factors, such as the stress of focusing on near objects when reading. For some children, this stress on the focusing system may cause their eyes to grow, and hence their myopia increases at a much faster rate. Bifocals could reduce that stress, and a study published in Archives of Ophthalmology in January 2010 showed that children wearing them had a 58 percent slower rate of progression of myopia, compared with children who wore traditional glasses with regular lenses that corrected for nearsightedness.

This study confirmed findings from an earlier one that showed a similar but more modest benefit from wearing bifocals. Meanwhile, some other studies have suggested that specially designed gas-permeable contact lenses may also slow myopic progression.

There isn't enough evidence yet to recommend that children with myopia wear bifocals or special contact lenses. Myopia is a very complex condition, and as the results of more well-designed studies get reported, we may be able to figure out a way to alter the course of this increasingly common form of visual impairment. — Stephen Taylor, O.D., Ophthalmic Consultants of Boston